Method of making strain gages



Jam-l3, 1948. E. E. SIMMONS, JR 2,

METHOD OF MAKING STRAIN GAGES Filed Nov. 18, 1943 2 Sheets-Sheet 1 INVENTOR EDWARD E. SIMMONS JR.

ATTORNEY Jan. 33, 1948. E. SIMMONS, JR

METHOD OF MAKING STRAIN GAGES 2 Sheets-Sheet 2 Filed NOV. 18, 1943 INVENTGR EDWARD E. SIMMON R.

ATTORNE Patented Jan. 13, 1948 UNITED STATES PATENT OFFICE METHOD or MAKING STRAIN GAGES Edward E. Simmons. Jr., Pasadena, Calif. Application November 18, 1943, Serial No. 510,827

2 Claims. (01. 201-63) This invention relates to an improved method of making electrical resistance devices, particularly of the strain gage type having a fine wire electrical resistance element adapted to be bonded throughout its effective length to a member subject to strain whereby variation in strain of the member causes a predetermined resistance change in the fine wire element, this application being a continuation-in-part of my copending application Serial No. 442,159, filed May 8, 1942, now Patent No. 2,350,073.

The character of the filament wire and the materials of which it may be made, among possible others, are fully described in my Patent No. 2,292,549, issued August 11, 1942, and hence it is not necessary to describe this wire in the present application except to point out that the wire is a continuous solid metallic filament whose diameter is preferably of the approximate order of .001 to .003", although the wire may be finer under certain circumstances. In any event the hair-like character of the wire renders it somewhat diflicult to arrange and support the wire in such a form that the completed gage can be conveniently handled and applied to a test member while at the same time having thesupporting medium of the gage sumciently thin to insure that the filament lies closely adjacent to the surface of the test member.

It is an object of my invention to provide an improved method for making electrical resist ance devices, and particularly strain gages, having a high degree of thinness and which can be formed relatively easily and economically with precision.

Other objects and advantages will be more apparent to those skilled in the art from the following description of the accompanying drawings in which:

Fig. 1 is a greatly enlarged cross-sectional perspective of one form-of gage filament embodying my invention before being applied to a test member;

Fig. 2 is a greatly enlarged cross-sectional view of my improved gage applied to the surface of a test member;

, Fig. 3 is a diagrammatic perspective showing a radiating heater for applying my improved gage to the test surface and also showing one form of my gage with a bow supporting means;

Fig. 4 diagrammatically illustrates a filament I type heating lamp adapted, if desired, to be used in applying my improved type of gage;

Fig. 5 shows one method by which my improved gage can be formed in a flat, rectangular form;

Fig. 6 shows a method by which the gage may be wound on an annular support;

Fig. 7 shows a further step in the process of manufacturing the gage of Fig. 5 after it has been subjected to pressure and heat;

a Fig. 8 is an enlarged fragmentary section of a gage showing the manner in Which the gage filament may be partially flattened to a ribbon-like form if sufficient pressure is applied during man ufacture;

Fig. 9 is a diagrammatic illustration showing a further method and arrangement for embedding the filament into it thermal plastic support together with providing external membranes for the gage;

Fig. 10 is a transverse section through a gage formed in accordance with the method of Fig, 9;

Fig. 11 is a further modified form of the gage employing a removable membrane which is temporarily used during the pressing operation;

Fig. 12 is an enlarged fragmentary sectional view of the gage showing the manner of connecting a lead to the filament; and

Fig. 13 is a sectional view showing a further method of forming any modified form of gage.

In the particular embodiments of the invention that are shown herein for the purposes of illustrating certain specific forms thereof, I have provided, as shown in Figs. leand2, a wire filament 1 of the type above referred to having an insulating layer 2 which, in turn,- is coated with a thermal softening cement or bonding layer 3 throughout the effective length of the gage. The insulating layer may be any usual wire enamel or silk thread covering while the cementing or bonding layer 3 consists of a thermal softening cement which by way of example and among many other possible types of thermal softening cements is specifically shown herein as an adhesive known as XL 5075 manufactured by Carbide and Carbon Chemicals Corporation. After a suitable length of the filament is enameled and coated with the thermal softening material the filament readily lends itself to use as an open type strain gage both in fabrication and application all as disclosed more fully in my copending application Serial No. 403,717, filed July 23, 1941, now Patent No, 2,393,714, issued Jan.

As disclosed insaidlatter application the filament may be attached to a suitable supporting frame and the insulating enamel apforth in parallel loops 4 on a suitable frame which is specifically shown as of an annular type 5. Regardless of how the open type filament may be supported at its-ends it is pressed against the strain subject member 6 in a desired position and heat is applied until the cement layer softens and bonds the filament to the surface of test member 6, the cement becoming hard when it cools which requires a very short interval of time compared to the hardening of a volatile solvent type cement as heretofore used. The insulating layer 2 is impervious to heat and serves to insulate the filament from the member in case the cement softens sufficiently to allow the insulated layer to come into contact with the test specimen. The cement may be softened by heating from an open gas flame if desired, although a radiating heater 1 is preferable such as a filament type reflector heat lamp 8, Fig. 4. The gage in this arrangement is of the type having an arched nexible name 5 with a filament l wound back and forth across the open end of the frame. The heating element 8 may be of such size as to be inserted Within the support 9 so as to overlie the filament In. The heating element ls diagrammatically shown at ll. This method provides a very rapid means for attaching gages to test members as the usual time of drying a volatile solvent cement is avoided, My improved gage and method of forming the same insures just the right amount of cement on the filament and thereby avoids the trouble of excess cement which if present may be unevenly distributed. The cement in this present modification is right where it is needed. The broad principle of the above described du plex coated wire filament employing a thermal plastic bonding element is essentially used in the modication of Figs. to 12. The principal feature of this form of gage is that the filament I3 is wound in a pre-formed member made of thermal plastic material of which there are a great many well-known types on the market one of which is said XL 5015. Subsequent to this winding operation the thermal plastic material and winding are placed as an assembled unit in a hot plate press between platens P and compressed to form a thin wafer with the filament embodied therein. For example, Figs. 5 and 6 show possible winding forms of suitable thermal plastic material upon which the filaments I3 and I4 are wound. The filament I3 is wound initially upon a relatively thick sheet of thermal plastic material of substantially rectangular f orm l5 while the filament i4 is wound upon an annular sheet l6. These forms may be of sufiicient thickness to make handling feasible, such for example as of the approximate order of 1%". After the forms are wound they are pressed between usual par-. allel platens of a press with application of heat to squash the plastic material into a very thin sheet l1, Fig. 7, the filament being forced to'be embedded therein. The excess flash is trimmed off along the dotted outline l8 after removal from the press. If desired, extreme pressures and hard faced press plates may be employed so as to flatten the filament strands into ribbon form as diagrammatically indicated at l9, Fig. 8. Since the use of strongly adhesive plastic materials may give trouble in adhesion tothe press plates, I may, if desired, employ thin membranes 20 and 2| placed over the wound form of Fig. 5 or 6 so as to form a barrier between the press plates and the plastic material l5 or iii. These membranes are 4 then bonded to the plastic material and then become part of the gage structure when the plastic material is squee'ied as shown in Fig 10. The membrane may be of suitable insulating material such as thin paper which serves as insulating means and this paper may be bonded to a test member by a thin layer of said thermal softening cement. In case it is not desired to have the membranes form part of the finished gage the membrane may be formed of metal foil which is removable by chemical etching after the pressing o eration. Membrane material suitable for this purpose is aluminum, tin, silver, etc. This process is particularly useful when it is desired to be able to directly thermally bond the gage to a test member. In this case the filament wire is not only pre-insulated as by enamel or silk covering material but is also embedded in a strongly adhesive film of thermal setting plastic bonding cement like "XL 5075" which may be softened so as to adhere to the surface of the test member throughout the length or the filament. In the application of the gage to a test surface suitable pressure is applied as by pressing down on the plastic medium, throughout the filament length, with a suitable block of wood or metal or a pallet knife, these being applied immediately after the heating source has been removed. In this connection one pressing membrane, such as 20, may be nonetchable such as paper so that no particular care will have to be exercised when etching off the single metallic membrane. The metallic membrane left on the plastic filament carrier will, of course, be placed on top when the gage is bonded to the test specimen thereby preventing the pallet knife or pressure block from adhering to the plastic material during application of the gage to the specimen. If desired, terminal leads 25 may be attached to filament 26 by bending a loop of the lead around the filament, This loop would be cold bonded to the filament during the pressing operation of Fig. 9, the cold bonding operation being particularly effective for metals such as lead, silver-and gold.

The term plastic material as used herein includes thermal setting plastics such as Bakelite which require heat and pressure to polymerize the same. The material would be partially unpolymerized when first wrapped. During heating it would become sufliciently soft to allow the filament to be embedded thereto under the pressure. The subsequent cooling would bond the filament throughout its length to the plastic.

The principle herein broadly disclosed is also applicable to gages of the type shown in my said copending application wherein a. filament is bonded to a piece of paper to form a gage unit. Heretofore this unit has been bonded to a test surface by a solvent type cement applied in wet form but then'attachment of the unit to the. test surface is I made by simultaneously heating and applying pressure'tothe-gage on thetestgsurface so as to soften the hardened thermal plastic material whereupon its subsequent rehardening bonds the gage to the test. surface. If the thermal plasticv cement material is Duco household cement" the gage may be applied by application of heat and pressure as just described or, if desired, the gage may be attached by applying a cement solvent solvent layer to produce adhesion through resolution of thecement and subsequent drying. In this manner the cement is an integral part of the gage unit applied thereto at the time of gage manufacture so that the use can quickly apply the relatively thin solvent to the test surface. On the other hand, the same gage lends itself to attachment by use of heat and pressure if the user prefers this method. vString gages of the type shown in Figs. 1 and 2 as well as any other open type gage could be attached either by application of heat and pressure or by placing the open gage filament in contact with the test member and then spraying or brushing the solvent over the open filament structure to produce adhesion by resolution and drying of the solvent cement such as Duco household cement which was applied as a second or duplex layer over the impervious insulating layer on the gage filament. That is, the insulating layer and duplex adhesion layer would form a part of the original filament structure. A further arrangement along these lines consists of attaching the filament 29 to a thin sheet of mica 30 with an irreversible baking cement 3i, such as Bakelite, which in turn is covered with either a thermal plastic bonding material 32 or a solvent bonding cement. The back of the mica sheet is permanently cemented to a stiff metallic washer 33 for ease in handling. This unit forms a sandwich which is placed with bonding material 32 against a test member and bonded by heatin and pressure. After cooling the heavy backing layer is split 0115 through one of the splitting planes of the mica 30.

While several bonding materials have been mentioned, it is, of course, obvious that there are other equivalent materials isofar as their physical 7 properties are necessary in my present invention.

'For instance, XL5075 is a polyvinyl acetate-chloride mixture, but butyl acetate would be equally satisfactory. XL5075 has the property of irreversibility of heating, that is, once it has been bonded by heating it hardens due to polymerization so that a second heating produces much less softening. Such material is applied to the filament in liquid form through the use of wellknown solvents and is dried at sufficiently low baking temperature to evaporate the solvent and form a coating film. For subsequent bonding a higher temperature is applied.

- From the foregoing disclosure of the several modifications, it is seen that I have provided an extremely simple and yet highly effective gage in which the supporting medium for the filament is not only thermally bonded thereto throughout the length of the filament but also serves as a" bonding medium which may quickly and easily bond the filament to a test member merely by applyin an external source of heat to the thermal plastic medium which will quickly cool so as to harden or set thereby avoiding the usual period required where volatile solvent cements are employed. The thermally effected bond between the filament, plastic material and the test surface is suflicient to transmit strains from the test member to the filament so as to accurately produce a corresponding change in filament resistance and to faithfully repeat alternate test' strains. The filaments in all forms may be preenameled so that this enamel together with the thermal plastic coatin forms broadly a duplex coated filament, although the enamel might be omitted under certain circumstances where there is little danger of the filament contacting the specimen when the thermal plastic material is initially softened during application of the gage. In addition, the filament may be easily handled during winding and yet it is thoroughly bonded throughout its length to the supporting medium which may be formed into a very thin and flexible strip. The thinness of the gage is conducive to. rapid thermal plastic action during application of the gage to the test member and is conducive plastic material.

It will of course be understood that various changes in details of construction and arrangement of parts may be made by those skilled in the art without departing from the spirit of the invention as set forth in the appended claims.

I claim:

1. The method of manufacturing an electrical resistance device of the type having a continuous solid resistance filament consisting of wrapping said filament around a piece of plastic material, then placing an etchable membrane over said material, and finally applying heat and pressureto the membrane and material so as to embed the filament in the plastic material.

2. The method of manufacturing an electrical resistance device of the type having a continuous solid resistance filament consisting of wrapping said filament around a piece of plastic material, then placin an etchable membrane over said material, applying heat and pressure to the membrane and material so as to embed the filament in the plastic material, and then removing said membrane by etching the same. 1

EDWARD E. SIMMONS, JR.

REFERENCES CITED The following references are of record in the me oi this patent:

UNITED STATES PATENTS OTHER REFERENCES General Radio Co., Catalog K (1939 issue), copyright 1938, pages 17 and 35.

De Forest et al., Technical Notes-NACA #744, Jan. 1940 (pages 4 and 10) Glenn L. Martin Co., Engg. Report #1526, Dec. 65 17, 1941 (page is). r

to equal rapidity of cooling and hardening of the 

